Polyacetylene production

ABSTRACT

The present invention relates to coherent poly(acetylene) films and to a method of producing such polyacetylenes. The process comprises solvent casting a solution of a polymer of the general formula (V) derived from a precursor (IV), and transforming the pre-cast, polymer (V) into a poly(acetylene) (III) film and a by-product (VI) at a temperature between 20° and 200° C. at reduced pressure. The poly(acetylene) thus formed has a substantially higher density than the poly(acetylene) polymers produced hitherto. The morphology of the poly(acetylene) produced is that of a thin, coherent solid film with no voids and, no basic structural units are visible even at a magnification of 10,000 times. The polymers also have a lower crystallinity than those produced by conventional methods.

The present invention relates to coherent poly(acetylene) films and amethod of producing such polyacetylenes.

Poly(acetylene) is known to have highly desirable electrical conductingproperties. Such polymers are usually produced by direct polymerisationof acetylene gas in the presence of a catalyst, eg as described by Itoet al (J. Polymer. Sci. Chem., 12, pp 11, 1974). The polymers thusproduced have a relatively low density of around 0.4 g/cc and have amorphology which is an open, irregular, fibrillar structure with randomorientation of the fibrils. The conductivity of such polymers hashitherto been improved by appropriate chemical doping. The morphology ofpolymers produced hitherto offers an advantage with respect to the speedof chemical reactions such as doping. However, due to the high surfacearea which is an inherent characteristic of such a morphology, thepoly(acetylene) is also highly susceptible to oxidative degradation.Moreover, the open and irregular morphology of the polymer and therandom orientation of the fibrils makes doping of specific areas of thefilm with well-defined edges, which is the basis of the semi-conductorindustry, virtually impossible. Such polymers are also infusible andinsoluble in conventional solvents thereby making it difficult tofabricate isotropic and anisotropic articles therefrom. There is acontinuing need in industry for a poly(acetylene) which can be easilyand conveniently fabricated into articles of a desired shape, and whichcan be fabricated to possess a degree of chain alignment. This chainalignment increases the anisotropy of electrical properties.

More recently, Edwards and Feast (Polymer, vol. 21, June 1980, pp 595)have described a method of producing poly(acetylene) (III) by firstpolymerising a precursor 7,8-bis(trifluoromethyl)tricyclo-[4,2,2,0² 5]-deca3,7,9-triene (I) using a catalyst in toluene and the precursorpolymer (II) so formed is spontaneously decomposed to a black productand 1,2-bis(trifluoromethyl)benzene. When the precursor polymer (II) washeated to 150° under a vacuum of 0.01 mm of mercury for 5 hours theauthors obtained a product which had an infra-red and Raman spectrumcorresponding to that of trans-poly(acetylene) although the elementalanalysis showed that only 96.3% of the fluorine had been removed. Whenheated for a further 3 hours at 210° C., 98.9% of the fluorine had beenremoved although the polymer would probably have been degraded by thisstage. The authors stated that this type of system is too labile forconvenient generation of poly(acetylene) and that they are investigatingrelated structures in order to find a more stable precursor.

It has now been found that a poly(acetylene) having a higher density anda markedly different morphology can be produced from the same or similarprecursor polymers under appropriate conditions.

Accordingly, the present invention is a coherent poly(acetylene) film.

According to a further embodiment, the present invention is a processfor producing a coherent poly(acetylene) (III) film comprising solventcasting a solution of a polymer of the general formula (V) derived froma precursor (IV), and transforming the pre-cast, polymer (V) into thepoly(acetylene) (III) film and a by-product (VI) at a temperaturebetween 20° and 200° C. under reduced pressure over a duration ofbetween 1 and 100 hours in an atmosphere inert to the precursor polymer(V) and to the poly(acetylene), wherein in the general formulae IV, Vand VI each of the groups R₁ and R₂

either (a) represent a radical selected from H, CX₃, C_(m) H_(2m+1) andCOOR₅ wherein X is a halogen atom, m has a value between 1 and 4 and R₅is an alkyl group with 1-4 carbon atoms,

or (b) form together with the respective carbon atoms to which they areattached a benzene nucleus,

and

each of R₃ and R₄

either (c) represent H atoms,

or (d) form together with the respective carbon atoms to which they areattached to a benzene nucleus. ##STR1##

Specific examples of the groups R₁ and R₂ are a trifluoromethyl group,an alkyl group or an alkyl carboxylate group, especially a methylcarboxylate group.

More specifically the precursor (IV) may be a compound in which R₁ andR₂ are each a trifluoromethyl group, and R₃ and R₄ are each a hydrogenatom. Such a compound is shown in formula I above.

The precursor polymers (V) used in the solvent casting step may beproduced by conventional means e.g. that described in the paper byEdwards and Feast referred to above. The polymerisation of theprecursors (IV) is suitably carried out in the presence of a tungstenhexachloride/tetralkyl or aryl tin (1:2 w/w) or titaniumtetrachloride/trialkyl or dihaloalkyl aluminium (1:2 w/w) catalyst atambient temperatures and pressures. Depending upon the tendency of theprecursor polymer to transform to poly(acetylene), it must be stored ata temperature low enough to slow down this transformation. For example,the precursor polymer (II) is suitably stored at relatively lowtemperatures suitably below -10° C., preferably below -20° C. andtypically -26° C. to prevent the premature transformation thereof intopoly(acetylene). At this temperature the precursor polymer of theformula (II) is stable for at least 14 months. Where R₁ and R₂ representa benzene ring and R₃ and R₄ represent hydrogen atoms when the precursorpolymer may be conveniently stored at room temperature at whichtemperature it is stable for at least six months.

For solvent casting, the precursor polymer (V) is preferably dissolvedin an organic solvent to a concentration which, for a given depth ofsolution, gives the desired thickness of the required shape. Thisconcentration is typically up to 100 g/l (approximately equivalent to ega film thickness of up to 100 microns). The precursor polymer issuitably cast from an organic solvent selected from acetone, chloroform,ethyl acetate, toluene and xylene although solvents such as toluene andxylene are preferred for precursor polymers with aromatic substituents.

The precursor polymer (V) may be cast into any desired shape althoughshapes with a relatively high surface area e.g. a film or a fibrefacilitate the transformation reaction. During the solvent castingprocess it is most desirable to minimise moisture and/or oxygen contentof the system in order to produce a coherent film having the desirableproperties of conductance. It is most preferable to carry out thecasting in an atmosphere inert with respect to the precursor polymer (V)and the eventual polyacetylene film formed. The inert atmosphere issuitably provided by nitrogen or argon gas. The casting temperature maybe suitably adjusted to control the rate of deposition of the precursorpolymer from the solvent.

After casting, the temperature at which and the duration for which theprecursor polymer is heated to produce the poly(acetylene) film willdepend upon the nature of the substituents in the precursor polymer. Forexample, the precursor polymer is preferably heated at a temperaturebetween 20° and 200° C. for between 1 and 100 hours to produce thepoly(acetylene) film. The Table below illustrates the preferred rangesfor various substituents in the precursor polymer.

                  TABLE 1                                                         ______________________________________                                        Substituents                                                                  R.sub.1                                                                              R.sub.2  R.sub.3 R.sub.4                                                                             Temp °C.                                                                      Time (hours)                             ______________________________________                                        H      H        H       H     20-50  1-20                                     CF.sub.3                                                                             CF.sub.3 H       H      50-120                                                                              1-100                                    CO.sub.2 CH.sub.3                                                                    CO.sub.2 CH.sub.3                                                                      H       H      50-120                                                                              1-100                                    benzene ring                                                                              H       H       100-150                                                                              1-100                                      benzene ring                                                                              benzene ring                                                                              175-200  1-100                                        ______________________________________                                    

For instance, the polymer (II) is heated at a temperature below 150° C.,preferably between 50° and 120° C., under vacuum or in the presence ofan inert atmosphere, e.g. nitrogen, to transform the precursor polymer(II) into a coherent poly(acetylene) film. The heating procedure may becarried out for a period between 1 and 100 hours suitably between 10 and50 hours to form the poly(acetylene) polymer. The rate of heating issuitably between 1° and 10° C. per minute. The lower the heatingtemperature, the longer the duration of heating. The preferred rangesspecified relate to that needed to achieve a substantially completetransformation of the precursor polymer to a coherent film. For someuses partial transformation may be adequate and hence slight variationsoutside the preferred ranges may be acceptable. It should be noted,however, that effort to transform the precursor polymer below 20° C.results in a creased and wrinkled film whereas at temperatures above200° C. there is substantial degradation of the polymer and loss ofcoherence due to appearance of voids.

The poly(acetylene) thus formed has a substantially higher density thanthe poly(acetylene) polymers produced hitherto. For instance, thedensity of the poly(acetylene) (III) produced according to the presentinvention is approximately 1.0 g/cc whereas that of the poly(acetylene)produced according to the prior art methods is only about 0.4-0.5 g/cc.The calculated density of poly(acetylene) is 1.2 g/cc. The morphology ofthe poly(acetylene) produced according to the present invention is shownby scanning electron microscopy (SEM) to be that of a thin, coherentsolid film with no voids and, no basic structural units are visible evenat a magnification of 10,000 times. At such magnificationpoly(acetylene) produced by prior art methods reveals a clear fibrillarstructure. The polymers also have a lower crystallinity than thoseproduced by conventional methods. The resonance Raman spectra show thatthese poly(acetylenes) have shorter lengths of conjugated double bond insequence than those derived by prior art processes. Typically, thecoherent poly(acetylene) films of the present invention have a C═Cstretching frequency of 1480 cm⁻¹ which may be interpreted as a sequenceof more than 25 double bonds per chain whereas the poly(acetylenes) ofprior art have a C═C stretching frequency of 1460 cm⁻¹ which may beinterpreted on the same basis as a sequence of at least 75 double bondsper chain. X-ray diffraction and electron diffraction spectra also showthe poly(acetylenes) of the present invention to be of lowercrystallinity and hence distinct.

The conductivity of the pristine poly(acetylene) produced according tothe present invention is in the range of between 10⁻⁸ and 10⁻⁵ per ohmper cm.

The electrical properties of the poly(acetylene) produced according tothe present invention may be altered as desired by addition of suitabledopants known in the art. Examples of dopants include the halogens,fluorides of arsenic, and protonic acids. The dopants may be addedeither to the solution from which the precursor polymer (V) is cast orto the pre-cast polymer by diffusion thereof from a gas or liquid phase,electrochemical diffusion or by ion implantation techniques. Themorphology of the poly(acetylene) produced by the process of the presentinvention renders it particularly suitable for selective area doping,with a resolution which is better than 1000 Å. In comparison the fibrilsin conventionally produced poly(acetylene) give a resolution figurewhich rises to around 100 microns which is approximately 1000 timeslarger.

Upon doping, the conductivity of these films can be substantiallyimproved. For instance, by using iodine as dopant the conductivity ofthe coherent film may be improved to a value of between 1 and 20 per ohmper cm.

The process of producing poly(acetylene) according to the presentinvention also enables controlled chain-alignment of the molecules inthe polymer prior to the transformation reaction. The polymer (V) may becast on a flexible substrate or in the form of a free-standing filmwhich can be stretched as necessary to provide the desired chainalignment. Alternatively it can be cast from a solution in a shear fieldto achieve the desired chain alignment.

The process for producing poly(acetylene) according to the presentinvention is further illustrated with reference to the followingExamples.

EXAMPLE 1

A solution of 1 g of the precursor polymer (II) dissolved in 25 ml ofacetone was prepared. Approximately 4.5 ml of this solution was castonto a 3 in. diameter silicon slice and the solvent allowed to evaporateunder a reduced pressure of nitrogen (200 mm of Hg pressure) at roomtemperature (about 25° C.) The pressure was then further reduced to lessthan 1 mm of Hg pressure, the temperature raised to 80° C., andmaintained for 10 hours. The fawn film gave way to a black, shiny filmof poly(acetylene). Examination of the film in the scanning electronmicroscope showed a solid, coherent film of 20±2 microns thickness. Thefilm had a measured density of about 1.0 g/cc. The conductivity wasfound to be approximately 10⁻⁶ (ohm cm)⁻¹.

EXAMPLE 2

A solution of 0.5 g of the precursor polymer (II) dissolved in 25 ml ofacetone was prepared. Approximately 0.5 ml of this solution was castonto a quartz plate and the solvent allowed to evaporate under anitrogen atmosphere at room temperature. The pressure was then reducedto less than 1 mm of Hg pressure and the sample maintained at roomtemperature (about 25° C.) for 24 hours. It was then heated up to 80° C.and maintained at that temperature for 15 min before cooling back toroom temperature. A film was formed, in which 90% of the precursorpolymer had been converted to a coherent polyacetylene film. Theconductivity of the film at room temperature was then measured and foundto be 3.5×10⁻⁶ (ohm cm)⁻¹. The density of the film was about 1.0 g/cc.The sample was then subjected five times, still under vacuum, to atemperature cycle of between 3° C. and 80° C. by heating at the rate of1° C. per minute to 80° C. and maintaining at that temperature for 20minutes before cooling down to start the next cycle. It was exposed toiodine vapour and doped to a molar ratio level of 17±4% with respect tothe polymer and represented by the empirical formula (CHI₀.17)_(x). Theconductivity was measured to be 0.1 (ohm cm)⁻¹ at 50° C. On examinationthe thickness of the film was found to be 20±8 microns.

EXAMPLE 3

Example 2 was repeated except that the precursor polymer was heated at80° C. for 2 hours and the conductivity of the coherent polyacetylenefilm (CHI₀.25)_(x) produced was 15(ohm cm)⁻¹ at 23° C.

EXAMPLE 4

A precursor polymer (V) in which R₁ and R₂ represent a benzene ring andR₃ and R₄ each represent a hydrogen atom was dissolved in ethyl acetate(0.75 gm/25 ml) and heated at 120° C. for 10 hours to give a coherentpoly(acetylene) film.

EXAMPLE 5

Kinetic analyses were carried out on various precursors using aDifferential Scanning Calorimeter and software which enabled analysis ofthe transformation reaction from precursor polymer to a poly(acetylene)film. The data was related to other spectroscopic analyses to optimisethe transformation. The data obtained is tabulated below:

                  TABLE 2                                                         ______________________________________                                                               Transformation                                                                (Heating Rate                                          Substituents in the    2.5° C./min)                                    precursor polymer      Temp range                                             R.sub.1                                                                              R.sub.2   R.sub.3  R.sub.4                                                                              From  To                                     ______________________________________                                        H      H         H        H      15     60                                    CF.sub.3                                                                             CF.sub.3  H        H      50    100                                    CO.sub.2 CH.sub.3                                                                    CO.sub.2 CH.sub.3                                                                       H        H      50    105                                    benzene ring H        H        90    160                                      benzene ring benzene ring  180      220*                                      ______________________________________                                         *Degradation of poly(acetylene) sets in above 200° C.             

EXAMPLE 6

A solution of 0.25 g of the precursor polymer (II) dissolved in 25 ml ofacetone was prepared. Samples for spectroscopic analysis were preparedin two ways:

(a) Transmission Electron Microscope grids coated with a thin layer ofcarbon were immersed in the solution, quickly dried and heated at 80°for 2 hours. They were then examined in a Transmission ElectronMicroscope, by electron diffraction techniques. They showed a diffusering in contrast to poly(acetylene) produced by conventional techniques[described by (i) Shirakawa, H. and Ikeda, S., Polymer Journal (1971),Vol 2(2), p 231 and (ii) Luttinger, L. B., Journal of Organic Chemistry,(1962), Vol. 27, p 1591. These films were produced at -78° C. and thenisomerised at 150° C. for 5 hours] which show a pattern of sharp rings.This indicates substantially reduced crystallinity for thepoly(acetylene) produced according to the present invention.

(b) The precursor polymer was cast from this solution on the inside of aflask. The precursor was transformed at 80° C. for 4 hours.

(c) The sample was examined by Resonance Raman Spectroscopy, whilststill in the flask under vacuum. When irradiated with a laser line of676.4 nm the sample gave a C═C stretching frequency at 1480 cm⁻¹. Thiscompares with a value of 1460 cm⁻¹ for poly(acetylene) prepared byconventional techniques, and indicates markedly shorter conjugationlengths.

(d) The film was removed from the flask under nitrogen and examined byX-ray diffraction. The trace obtained showed a broader, less intensepeak at higher lattice spacings than those obtained from conventionallyproduced poly(acetylene) [cf (i) Shirakawa et al and (ii) Luttinger Loc.Cit.]. This confirms the electron diffraction experiment suggestingsubstantially reduced crystallinity.

EXAMPLE 7

A solution of 0.5 g of the precursor polymer (II) dissolved in 25 ml ofdeoxygenated acetone was prepared. Approximately 10 ml of this solutionwas placed inside a round bottom flask. It was spun around the innersurface of the flask by gentle rotation of the latter. This process wascontinued until all the solvent had evaporated under a nitrogenatmosphere at room temperature. The pressure was then reduced to lessthan 10⁻³ torr as it was heated up to 80° C. and maintained at thattemperature for 2 hours before cooling back to room temperature. A filmof about 15 microns thick was formed. A portion of this film (5.6 mg byweight) was used for the iodine weight uptake experiments using aquartz-fibre coil spring (extension 1 mm/mg). The diffusion coefficientof iodine in the film thus obtained was approximately 10⁻⁴ cm² s⁻¹,which corresponds to a geometric resolution of less than 20 microns. Incomparison the geometric resolution of the conventionally producedpoly(acetylene) [(as described by Berniere, F. et al, in Journal ofPhysics and Chemistry of Solids, Vol 42, pp 649-654f, (1981)] was higherthan 5 mm.

EXAMPLE 8

A solution of 2 gm of the precursor polymer (II) dissolved in 20 ml ofacetone was prepared. Approximately 1 ml of this solution was cast ontoan aluminium scanning electron microscope stub. The temperature wasquickly raised to 150° C. and maintained for 5 hours under a vacuum of0.01 mm of Hg. The edges of the film became detached from the stub andcurled up. One such section was fractured and the broken edge examined.It showed significant voiding.

EXAMPLE 9

A solution of 5 gm of the precursor polymer (II) dissolved in 20 ml ofacetone was prepared. It was cast onto a salt-plate and kept under avacuum of 0.01 mm of Hg at room temperature (approx. 22° C.) for 200hours. The film appeared lumpy and shrivelled. Infra-red examination ofthe film showed less than 70% loss of hexafluoroortho-xylene.

EXAMPLE 10

Approximately 5 ml of the precursor polymer solution of Example 7 wascast on a glass slide. It was first evacuated at room temperature underless than 10⁻³ torr to remove the solvent (acetone). The sample slidewas then sealed onto the sample holder of a Perkin Elmer UV/VISspectrophotometer. The latter was evacuated to a vacuum of less than10⁻³ torr at 23° C. The sample was yellowish in colour and an opticalspectrum (0.5-5.5 eV) was first obtained at 23° C. The absorption edge(ie optical bandgap) of this spectrum was 2.0 eV. The sample was thenheated at 65° C. for 5 hours and the optical scanning was repeated everyhour. As the appearance of the polymer changed from yellow, throughorange, red and brown to eventually black, metallic lustre, the opticalbandgap decreased from 2.0 eV to 1.5 eV.

We claim:
 1. A coherent poly(acetylene) film, characterized in that saidfilm is a solid film with no voids and with no basic structural unitsbeing visible under a scanning electron microscope at a magnification of10,000 times.
 2. A coherent poly(acetylene) film according to claim 1characterised in that said film has a density of at least 1 g/cc.
 3. Acoherent poly(acetylene) film according to claim 1 characterised in thatsaid film in pristine form has a conductivity of between 10⁻⁸ and 10⁻⁵per ohm per cm.
 4. A coherent poly(acetylene) film according to claim 1characterised in that said film has a sequence of no more than 25conjugated double bonds per chain as determined by resonance RamanSpectroscopy.